The advantages of using single crystal semiconductors such as Si as a mechanical material have long been recognized. For example, its strength and high intrinsic quality factor make it attractive for micromechanical resonant devices. It is readily available as an integrated circuit (IC) substrate and can be processed using methods developed by the IC industry.
Thick Si devices also can have advantages over thinner ones for many applications. When capacitive transduction is used to drive or sense motion in a micromechanical device, large capacitive plates with small gaps between them are desired to increase capacitance so that high sensitivity can be achieved. For laterally resonant devices, this translates into a thick structure. Thick structures can also be advantageous for inertial sensing applications where large masses are required to respond to small inertial forces.
One of the obstacles in the production of single crystal Si micromechanical devices has been the ability to integrate electrical circuitry with the micromechanics using a simple fabrication process. Integration of circuitry with micromechanics can provide a number of advantages for many sensing and signal processing applications. Often the output of micromechanical devices is a very small electrical signal. The difficulty in reading out a small output signal can limit the sensitivity or signal-to-noise ratio in many devices. This signal is usually buffered or amplified so that it can then be processed by the rest of the electronic system. When the signal processing is done on a separate chip from the micromechanics, the signal must travel through bond pads, bond wires, and external packaging structures which have large parasitic capacitances associated with them. This further limits the signal which can be read out. However, if the signal processing circuitry can be included on the same chip as the micromechanical structure, smaller signals can be amplified and conditioned, and even converted to digital signals so that when they are passed off chip, they are not degraded significantly by off-chip parasitics.
A number of technologies have been developed which integrate micromechanics and electrical circuitry. Many use surface micromachined polycrystalline Si micromechanical elements due to this material's availability, often used as transistor gates, in electrical circuitry. The use of thin polycrystalline Si layers provides flexibility in geometry, transducer axis selection, anchoring, and number of structural layers.
However, the use of polycrystalline Si brings with it some limitations in design. Polycrystalline Si is typically deposited at relatively high temperatures. As such, when it cools down to room temperature, stress gradients developed in the film due to mismatches in thermal expansion coefficient can cause a released micromechanical device to bend. Therefore, deposition conditions must be carefully selected and monitored in order to produce a polycrystalline film with low stress. Also, film thicknesses are typically limited to a few micrometers due to the long depositions times. However, recent work has demonstrated that thick polycrystalline Si films can be grown in acceptable times with useful properties.
There are various processes used to fabricate single-crystal Si micromechanical devices but most are difficult to integrate with conventional circuit processes, or expensive Si on insulator (SOI) starting wafers are required. There have been a number of successful efforts to integrate single crystal Si micromechanical structures with conventional circuitry and all have several advantages and drawbacks.